We propose to investigate how genes control developmental pathways by studying the engrailed locus of Drosophila. This genetic locus controls the fundamental segment and compartment segregation of the early embryo. A better understanding of its genetic structure, developmental role, and biochemical products will provide insights into basic regulatory mechanisms of development. This work is divided into two projects. The first explores the role of compartments in the development of the segments of the head and genitalia and in the development of the internal organs of the fly. This study will complete our description of compartmentalization and its control by the engrailed locus. This will indicate how generally compartment characterize the development of different structures and will define the relationship between compartmentalization and segmentation. The second initiates an investigation into the biochemical mechanisms that regulate developement. We will isolate the engrailed DNA segment and identify its transcription and translation products. This study should indicate how developmental loci such as engrailed functin by identifying the products of the locus and it should indicate how mutational lesions affect the expression of complex genes such as engrailed by examining the effects of mutations on the synthesis of these products.